Support waiting in smp_cond_load_relaxed_timeout() via
__cmpwait_relaxed(). To ensure that we wake from waiting in WFE
periodically and don't block forever if there are no stores to
ptr, this path is only used when the event-stream is enabled.

Note that when using __cmpwait_relaxed() we ignore the timeout
value, allowing an overshoot by up to the event-stream period.
And, in the unlikely event that the event-stream is unavailable,
fallback to spin-waiting.

Define CPU_POLL_RELAX_WAITS to state that we have a waiting
implementation. (Non-production environments might not have
arch_timer_evtstrm_available() but we don't care about that
configuration.)

Note that with this we have enough to define ARCH_HAS_CPU_RELAX to
indicate that we support an optimized implementation of
cpu_poll_relax(). However, defer defining ARCH_HAS_CPU_RELAX as that
enables polling based C-state handling, which really needs
TIF_POLLING_NRFLAG.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Suggested-by: Will Deacon <will@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
Notes:
  - instead of defining SMP_TIMEOUT_POLL_COUNT to 1, just state that
    we have a waiting implementation by defining CPU_POLL_RELAX_WAITS.
    - update comment to that effect.

 arch/arm64/include/asm/barrier.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 9495c4441a46..d186a4558776 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -12,6 +12,7 @@
 #include <linux/kasan-checks.h>
 
 #include <asm/alternative-macros.h>
+#include <asm/vdso/processor.h>
 
 #define __nops(n)	".rept	" #n "\nnop\n.endr\n"
 #define nops(n)		asm volatile(__nops(n))
@@ -219,6 +220,25 @@ do {									\
 	(typeof(*ptr))VAL;						\
 })
 
+/* Re-declared here to avoid include dependency. */
+extern bool arch_timer_evtstrm_available(void);
+
+/*
+ * In the common case, cpu_poll_relax() sits waiting in __cmpwait_relaxed()
+ * for @ptr value to change.
+ *
+ * State this by defining CPU_POLL_RELAX_WAITS which enables a time-check
+ * optimization in smp_cond_load_{relaxed,acquire}_timeout().
+ */
+#define CPU_POLL_RELAX_WAITS
+
+#define cpu_poll_relax(ptr, val, timeout_ns) do {			\
+	if (arch_timer_evtstrm_available())				\
+		__cmpwait_relaxed(ptr, val);				\
+	else								\
+		cpu_relax();						\
+} while (0)
+
 #include <asm-generic/barrier.h>
 
 #endif	/* __ASSEMBLER__ */
-- 
2.43.7